The present invention relates to environmental control systems. More specifically, the present invention relates to an environmental control system including an ozone-destroying catalytic converter.
A commercial aircraft usually includes an environmental control system for providing a stream of cooled, conditioned air to an aircraft cabin. A typical environmental control system receives compressed air such as bleed air from a compressor stage of an aircraft gas turbine engine, expands the compressed air in a cooling turbine and removes moisture from the compressed air via a water extractor.
Toxic ozone in the compressed air becomes an issue when an aircraft is cruising at altitudes that exceed 20,000 feet. To reduce the ozone to a level within satisfactory limits, the environmental system is provided with an ozone-destroying catalytic converter.
There are a number of desirable characteristics for an ozone-destroying catalytic converter of an aircraft. These characteristics include a) high efficiency of ozone conversion at bleed air operating temperature; b) good poison resistance from humidity, sulfur compounds, oil, dust, and the like, which may be present in the compressed air (for long life and minimum system overhaul and maintenance costs); c) light weight to minimize system parasitic load; d) high structural integrity of catalyst support under extreme heat and/or vibration shock, which may arise during normal flight conditions (also for long life and minimum system overhaul and maintenance costs); and e) high mass transport efficiency with low pressure drop.
An ozone-destroying catalytic converter with a metal core may be washcoated with a slurry of a water-based silica sol and a refractory metal to form an undercoat layer followed by an overcoat layer of alumina oxide. Both layers may then be catalyzed directly by dipping the washcoated core in a catalyst solution having strong acidity. However, the strongly acidity can cause corrosion of the metal core, especially if the core is made of aluminum.
The overcoat layer may be pre-catalyzed and then washcoated onto the core. Using a pre-catalyzed layer can prevent corrosion during the catalyzing process.
Applying the pre-catalyzed overcoat layer can be problematic. For example, it is difficult to control the uniformity of washcoat layer thickness. Unevenness of the layer thickness can cause a pressure drop across the catalytic converter.
Another problem with the pre-catalyzed overcoat layer is poor catalyst utilization efficiency. Washcoating the pre-catalyzed metal oxide can render certain fractions of the catalytic site inaccessible due to the shielding of the binder material. Furthermore, the surface area provided by the undercoat is not utilized to extend the catalyst lifetime. Since poisons in the compressed air can reduce the efficiency of conversion, lifetime and efficiency of the catalytic converter is further reduced because of the poor catalyst utilization efficiency.
Another potential problem with water-based washcoat layers is its limited mechanical durability. A catalytic converter for a commercial aircraft is subjected to high temperatures and large temperature swings (e.g., between 150xc2x0 F. and 500xc2x0 F.) during normal flight operation. The catalytic converter is also subjected to high vibrations during normal flight operation. These harsh conditions can cause the washcoat layer to flake off. Consequently, operating life of the catalytic converter is reduced.
According to one aspect of the present invention, an ozone-destroying catalytic converter comprises a core; an anodized surface layer formed from a portion of the core; a washcoat layer on the anodized layer; and an ozone-destroying catalyst impregnated in the washcoat layer. The combination of the anodized and washcoat layers offers many advantages. The anodized layer provides a support for the catalyst and a corrosion barrier that prevents a catalyzing reagent from attacking the core during catalyst impregnation. Therefore, the catalyst can be impregnated after formation of the washcoat layer to provide maximum catalyst utilization and lifetime. The anodized layer significantly improves the binding strength between the core and the washcoat layer, which allows the washcoat layer to withstand high temperatures, large temperature swings and high vibrations such as those occurring during normal aircraft flight conditions. The anodized layer also provides additional surface area and, therefore, increases the efficiency of ozone conversion and mass transport.
According to another aspect of the present invention, the washcoat layer may be formed by creating a slurry including a refractory metal oxide and an organosiloxane resin in monomeric or polymeric form. The refractory metal oxide may be partially hydrated. The core is dipped in the slurry and the resulting washcoat is dried. Such a slurry dries faster than slurries that include water-based binders. The faster drying allows the washcoat layer to be applied more uniformly than a washcoat layer formed from a slurry that includes a water-based binder. Thus, thickness and roughness of the surface can be controlled better.
The dried washcoat is then cured and calcined. If the washcoat layer is applied to an anodized layer of the core, cross-linking of the chemical bonds between metal oxide particles, anodized surface and organosiloxane resin occurs during the curing and calcination. This cross-linking results in a washcoat layer having significant mechanical and thermal strength. Consequently, the washcoat layer is free from flaking.